Demands have been increasingly made to reduce the thickness of display devices such as television sets and personal computer monitors for saving spaces. As such display devices that can be slimmed, plasma display (PDP) devices, field emission display (FED) devices, and liquid crystal display devices containing a backlight and a thin liquid crystal panel have been increasingly developed.
The plasma display devices use a plasma display panel as a light emitting device. The plasma display panel includes micro discharge spaces each containing a rare gas, and a phosphor layer arranged in each micro discharge space. The plasma display panel is so configured as to emit light in the visible region (visible radiation) by exciting a phosphor in the phosphor layer upon irradiation with ultraviolet light emitted in a negative glow region in the micro discharge space as an excitation source and thereby accelerating light emission of the phosphor. The ultraviolet light has wavelengths at 147 nm and 172 nm when xenon gas is used as the rare gas. The plasma display deices display images by controlling the intensity (quantity) and color of the light emission of the plasma display panel.
The liquid crystal display devices include a backlight and a liquid crystal panel, which liquid crystal panel contains a pair of electrodes and a liquid crystal sandwiched between the at least one pair of electrodes. They display desired images in which the quantity and/or color of light emitted from the backlight is controlled by the liquid crystal panel. Linear tubular white fluorescent lamps with a fluorescent material inside thereof are generally used as the backlight.
Details of these technologies can be seen, for example, in Japanese Patent Laid-Open No. 2003-132803, No. 2003-142004, No. 2003-242892 and No. 2003-346660; “Phosphor Handbook” (in Japanese) edited by Phosphor Research Society (in The Electrochemical Society of Japan), III, Chapter 2, pp. 219–223, (1987), Ohmsha, Ltd.; IDW '00 Proceedings of The Seventh International Display Workshops, pp. 639–642 (2000); technical report of IEICE, EID 2003–69(2004-01), pp. 45–48; and “FLAT-PANEL DISPLAY 2003 (Jitsumu-hen)” (in Japanese), Part 7.1, 210–217, (2002), Nikkei Business Publications, Inc.
Light emitting devices for such plasma display devices and FED devices, and backlights for liquid crystal display devices should have higher and higher performance. The properties of these devices are affected by the designs, configurations and materials for the devices and are significantly affected by phosphors for use in the light emitting devices.
Red-, blue- and green-emitting phosphors are used as the phosphors for plasma display devices, of which blue-emitting phosphors generally contain an aluminate phosphor (BaMgAl10O17:Eu; hereinafter briefly referred to as BAM). BAM exhibits good properties in its light emission but is susceptible to deterioration. Namely, BAM exhibits insufficient reliability, has a short life and thereby must have improved stability and a longer life. It must also have a high color purity and a higher photoluminescent brightness for higher performance of light emitting devices and display devices.
The lamps and backlights for liquid crystal display devices should be free from mercury for higher luminance on display surface and for reducing environmental burdens. As a possible candidate, flat-panel fluorescent lamps using rare gas discharge have been developed as the backlights for liquid crystal display devices. Such flat-panel fluorescent lamps using rare gas discharge generally use phosphors capable of being excited upon irradiation with vacuum ultraviolet light to thereby emit light. These phosphors must efficiently emit light with high luminance and color purity as a result of excitation by vacuum ultraviolet light and have a long life.
Blue-emitting phosphors with high luminance, color purity and reliability have been developed as an alternative to BAM. For example, silicate phosphors, such as Ca1-xMgSi2O6:Eux (hereinafter referred to as CMS), have been proposed as a blue-emitting phosphor that can be used in PDP devices and rare gas discharge lamps and have higher reliability and longer life than the conventional blue-emitting phosphor, BAM.
Although emitting light with high luminance and satisfactory color purity upon application of ultraviolet light at wavelengths of around 147 nm as an excitation source, CMS has substantially no excitation band at wavelengths of 160 nm to 210 nm, namely, shows a significantly low emission intensity upon excitation by vacuum ultraviolet light at around 172 nm (Xe2 molecular line). The Xe2 molecular line plays an important role in plasma display panels.
In addition to the above, the configurations of panels in plasma display devices have been investigated and improved for high emission efficiency thereof. For example, a “high Xe-content” technology in plasma display devices has been developed, in which the mole fraction of Xe gas in a discharge gas is increased to about 4% or more to thereby positively use the Xe2 molecular line and to increase the emission efficiency of the plasma display devices.
CMS originally having a low availability of the Xe2 molecular line, however, does not sufficiently contribute to higher efficiency of the plasma display devices. More specifically, CMS shows a low emission efficiency upon excitation by ultraviolet light at 172 nm and shows insufficient properties in its luminance, even if the amount of ultraviolet light at wavelengths of around 172 nm. For use as an alternative to BAM and for higher efficiency of plasma display devices, CMS must be further improved in its emission efficiency upon excitation by ultraviolet light at 172 nm.
Accordingly, an object of the present invention is to improve the life of a phosphor, typically a blue-emitting phosphor, which is for use in excitation by vacuum ultraviolet light typically in plasma display devices. Another object of the present invention is to provide a light emitting device which has a sufficiently long life (duration during which the light emitting device can be used in general manner) and sufficient luminance properties using the phosphor. Yet another object of the present invention is to provide an image display device which has a sufficiently long life (duration during which the light emitting device can be used in a regular manner) and sufficient luminance properties and exhibits satisfactory performance typically in color purity using the light emitting device.
Another object of the present invention is to improve the luminance properties of a silicate phosphor, as a possible solution to conventional problems in blue-emitting phosphors, typically upon excitation by the Xe2molecular line at a wavelength of 172 nm. Yet another object of the present invention is to improve luminance properties of a conventional light emitting device and a conventional display device using the light emitting device, and a next-generation light emitting device, such as a plasma display device, which actively uses excitation by the Xe2 molecular line at 172 nm and a plasma display panel image display device using the plasma display device.